1. Field of the Invention
This invention relates to physiological monitoring and diagnostic testing devices and methods. More particularly, this invention relates to a device and method for monitoring levels of a person's arm movements during sleep and using this data to compute the person's “sleep score” based upon these movements.
2. Description of the Related Art
Sleep deprivation is almost always a component of an unbalanced lifestyle. In our fast-paced, chaotic society, the average adult in the United States sleeps 6.9 hours per night during the workweek, as opposed to the ideal 8 hours. More than fifty percent of Americans suffer from insomnia several nights a week. Many don't wake up feeling refreshed, and many experience sleepiness during daily activities like reading, watching TV, riding in a car, or sitting quietly after lunch.
Few realize the price they pay for cutting themselves short when it comes to sleep. Sleep deprivation is unhealthy for and is clearly associated with a variety of problems. Without proper sleep, people: (a) are more susceptible to illnesses and have a greater chance of experiencing emotional and mental health problems, (b) have a lower frustration tolerance and may tend to overreact when stresses occur in their lives, and (c) have diminished capacities to concentrate, remember, learn and complete tasks. Thousands die each year in automobile accidents that are the result of someone falling asleep at the wheel. There is even evidence that without proper sleep we accelerate aspects of the aging process and shorten our life span.
Although most are aware of the importance of a good night's sleep, there presently is no easy method of gauging just how well we actually do sleep. For most people, awareness of the quality of their sleep (i.e., how soundly they slept through the night) without a means of measurement is difficult, if not impossible.
Presently, there are two validated medical methods available to measure a person's sleep behavior. These methods are however, not intended to measure the quality of night-to-night sleep, but to identify sleep disorders such as insomnia, sleep apnea, the restless legs syndrome (RLS) and periodic limb movements (PLM).
The first method, the medically accepted gold standard, comprises comprehensive sleep testing utilizing polysomnography (PSG). This medical procedure involves a full night testing in a sleep laboratory to monitor the temporal variations in the amplitude of the patient's sleep-impacted, physiological parameters, including: a continuous measure of the level of oxygen saturation in the arterial blood flow (SpO2), heart rate, upper respiratory airflow, thorax and abdomen respiration efforts, electroencephalograms (EEG; electrical activity of the brain), electro-oculogram (EOG; electrical activity related to movement of the eyes), and electromyograms (EMG; electrical activity of a muscle).
The PSG testing procedures are expensive as they are typically conducted in clinical settings by trained PSG technician in attendance. Recently, more limited PSG monitoring procedures utilizing at least four physiological parameters has been shown to provide a reliable means of screening for sleep disorders. Typically, these limited procedures are administered in the home setting but require an attending PSG technician for the entire sleep night. Even more recently, advancements in limited PSG recording have resulted in some variations where the person is taught by a sleep professional to self-apply the sensors and electrodes which are tethered to physiological recorders. The results are then viewed and evaluated by a sleep professional.
A second less complex method for assessing both sleep behavior (i.e., extent of sleep versus wake state during the sleep period) and screening for sleep disorders is by actigraphy means. Like PSG, this is a medical sleep testing procedure that requires the instrument to be cleared for use by the FDA and can only be used under the direction of a sleep professional. Additionally, since an actigraphy is typically used as tool for diagnosing sleep disorders and monitoring treatment of the disorder, the device can only be purchased by prescription.
Actigraphy methods utilize a wrist-worn actigraph recording device and associated sleep analysis software. The actigraph recorder, first described by Colburn et. al. in 1976 and later patented in 1982 (U.S. Pat. No. 4,353,375) captures arm movements by means of an accelerometer sensor. The device stores accumulated activity data, identified as activity counts, in pre-set time epochs of 30, 60 or 120 seconds. This information is then download to an external computer for graphical display and analysis.
The software for this analysis requires a complex, multi-pass analysis program. An example of this type of program was presented by the present inventors' at the 10th Annual Meeting, APPS, Abstract No. 309, 1996; where a three-pass, computer algorithm is used to score arm activity data (activity counts) recorded in thirty second epochs and downloaded from an actigraph device (i.e., an “ActiTrac” monitor which is manufactured by IM Systems, Baltimore, Md.).
The first pass in this analysis identifies whether one was in a sleep or wake state during a particular 30-second epoch according to the following formula:A+B+E≧K (a constant, which is set to 18), identified as a wake state epoch “<18, identified as a sleep state epoch                where:E=the activity count for the current epoch in question,A=the sum of the activity counts for the four epochs preceding the current epoch, B=the sum of the activity counts for the four epochs following the current epoch.        
The second pass in this analysis uses an algorithm that is designed to identify movement artifacts due to brief arousals and the sleep hysteresis effect. Artifacts are defined as any situation in which, for a given epoch: (A+B+E)≧18 and (A+B)=0. Identification of such events resulted in the activity count of the target epoch being re-coded as “0.”
The third pass in this analysis conducts a recalculation of sleep-wake states for those epochs affected by the second pass change, which would be the four epochs both preceding and following the current epoch.
The logic behind this sleep/wake scoring algorithm assumes that a subject is asleep when no arm movements are present. When a short burst of movement is detected, for example when the subject rolls over in bed, such activity is classified as an “arousal”. When there is a long burst of movement, the person is considered to be fully awake.
Thus, this sleep scoring program scores a fully asleep condition when there are three consecutive minutes of low arm activity, and conversely, the subject is assumed to be in the awake state when three consecutive minutes of high activity are detected. The final actigraphy sleep score compares the measured awake-time to the measured sleep-time and computes the results in terms of a sleep efficiency percentage (0-100%).
The less-than-desirable features of such actigraphy methodology include: 1) the equipment used is very expensive, a typical system including wrist recorder, download interface and analysis software ranges from approximately $2,000 (for an “ActiTrac” device from IM Systems) to $4,000 (for a “Mini-Motionlogger” from Ambulatory Monitoring, Ardsley, N.Y.), 2) its data must be downloaded to an external computer for analysis, 3) the use of the system requires administration and analysis by a skilled sleep professional, and 4) actigraphy recorders must be FDA cleared as class II devices, and can only be purchased by a sleep professional with a prescription, 5) its hardware is designed to detect sleep disorders by means of a sleep efficiency score, and 6) its equipment is not appropriate for home use by those who might wish to measure their night-to-night quality of sleep.
A need therefore exists for an alternative form of sleep monitoring that can provide the non-professional with an easy and inexpensive method of monitoring their own sleep performance in the privacy of their own bedroom. The present invention ideally serves this need.